When a structural element is subjected to a large amount of acceleration or of deceleration relative to the structure that carries it, and if said structural element is rigidly attached to its carrying structure, it tends to be torn off therefrom under loading from its own inertia. If said element is deformable, a portion of the energy due to the inertial load of the structural element is transformed into kinetic energy which can be absorbed by an absorber element.
In some cases, it can be necessary to limit the displacement of the structural element relative to the supporting structure. Otherwise, surrounding elements or elements supported by the structural element may be damaged.
In such cases, it is therefore essential to obtain the best possible ratio between energy absorption and displacement limitation.
This is a constant concern of the manufacturers of seats for passenger aircraft. In the event that the aircraft crashes, or that a large amount of acceleration or of deceleration is suddenly caused for any other reason, the potential energy of the inertial load of the seat and of its occupant must be attenuated by displacing the structures of the seat and by absorbing the resulting kinetic energy so as to prevent the seat and its occupant from being torn off from the floor of the cabin. However, such displacements must be limited so that occupant does not plough into the seat which is situated in front of said occupant. Current safety standards impose maximum displacements in various directions on several elements (e.g. the seatback, or the seat proper).
The systems currently used for obtaining the best ratio of energy absorption to displacement rely on hinged or deformable structures in which an energy absorption device provides rigidity in the normal operating position, and absorbs energy and controls displacement during a crash.
FR-A-2 612 151 describes an aircraft seat underframe provided with an energy absorption device comprising a rigid rod, one end of the rod being associated with a first element of the structure, and the other end being sharp and engaged in a block made of a plastics or equivalent synthetic material, so that the sharp end of the rod can penetrate into the block by cutting through the material of the block, once a certain value is reached for the force pressing the rod against the block. The block is associated with a second element of the structure, which element moves relative to the first element in the event of accidental sudden acceleration or deceleration. The device may operate either in traction or in compression. Such a device suffers from a certain number of drawbacks, such as its lack of compactness, and the difficulties and the high cost involved in obtaining a wide range of trip strength values and a wide range of energy absorption capacities. Furthermore, such devices have tolerances of plus or minus 10% on their strength values, and this prevents safety from being optimized with respect to tripping the device as a function of the use which is made of the underframe (weight of the seat to be supported, number of seats, position of the underframe relative to the floor of the aircraft and to the other seats, etc.).
U.S. Pat. No. 4,911,381 discloses an aircraft seat underframe provided with an energy absorption device which is equivalent to the above-described device, which suffers from the same drawbacks, and which comprises a rear leg hinged to a triangular bottom base.